Measurement of electrical circuit parameters, such as voltage, current, resistance, and power, is often required for monitoring a circuit for normal operation or for localizing a fault in the circuit. Many different types of instruments, varying over a wide range of price and complexity, are available to measure one or more of these parameters directly or to derive them from other measurements.
The simplest measurement devices are meters, which may be configured to measure voltage, current, resistance, or some combination of these parameters. An example of an ohmmeter is based on a d'Arsonval meter circuit, which consists of a meter in series with a battery and variable resistor. The meter has a pointer that moves in a manner proportional to current. The ohmmeter is calibrated by adjusting the variable resistor to show a zero resistance reading. Then when an unknown resistance is connected, the meter reflects the unknown resistance.
Digital multimeters combine a voltmeter, ammeter, and ohmmeter in a single device. The internal circuits of the multimeter convert the selected input variable to a dc voltage that is proportional to the measured value for the input variable. This dc voltage is converted to digital form and a numerical value is displayed.
Power has conventionally been measured with a wattmeter. A typical wattmeter has a pair of fixed current coils and a moveable coil. The current coils are in series with the load and have the same line current. The moveable coil is across the load and carries a needle that moves over a scale. Field strength of the current coils is proportional to the line current and in phase with it. The moveable coil produces a flux that is dependent on voltage. Because the force acting on the moveable coil is proportional to the time average of the product of the line current and the voltage, the needle deflection is proportional to power.
Other circuit parameters, such as impedance and reactance, may be derived from measured values. Various techniques for calculating these parameters are known in the art of circuit analysis.